This invention is related to Faraday rotation glasses and is more particularly concerned with a new Faraday rotation glass which exhibits a high Verdet constant and low susceptibility toward devitrification.
Some optical materials are themselves optically active, that is, they rotate the plane of polarization of polarized light passing through them. Glass and some other similar substances are devoid of this property in their normal condition, but may exhibit this property when placed in a strong magnetic field. This property is called the Faraday effect. The polarized light must traverse the substance along the magnetic lines of force. The direction of rotation of the plane of polarization is reversed if the field is reversed but is the same with respect to the observer whether the light is going or coming, so that a beam passing through the material in one direction and reflected back through has its rotation doubled. This is in contrast with the normal optically active material wherein the double pass through the optically active material serves to negate the rotational effect in the material.
Glasses exhibiting the Faraday rotation effect have found considerable use in various optical areas, for example, as high speed photographic shutters and in various laser optical systems. As the uses of laser materials have increased in recent years the interest in Faraday rotating materials has likewise increased.
It is known that cerium containing glasses produce the Faraday rotation effect and that the specific rotational capabilities of the cerium glasses vary essentially linearly with a concentration of the cerium ion. (See U.S. Pat. No. 3,711,264). However, many of these known glasses exhibiting the Faraday rotation effect are cumbersome and extremely difficult to fabricate, particularly on a large scale. Another common host glass system for Faraday rotation glasses is the heavy lead silicate. This system requires ceramic crucibles for melting. Consequently, the resulting glass is very inhomogeneous as the glass tends to attack the ceramic crucible. In addition, this attack may cause submicroscopic metallic or semimetallic inclusions within the host glass material. A second conventional system for Faraday rotation glasses is the terbium-aluminasilicate glasses. (See U.S. Pat. No. 3,484,152). These glasses require high melting temperatures and are very limited as to the size of the melt due to devitrification problems. Terbium metaphosphate glasses also require ceramic crucibles and also have a strong tendency toward devitrification. Rapid quenching of the melt of these glasses is essential and can only be achieved for specimens whose geometry allows the center of the glass specimen to cool rapidly. This necessarily limits the production of these glasses to thin discs. In addition, a wellcontrolled, reducing atmosphere is absolutely essential to obtain the proper valence of the cerium ions in the case of cerium metaphosphate glasses.